Detail this asset from Unity Store: Original Link
This is a paid asset, but now you can download Terrain Spam Free.
Terrain Spam v1.0
Terrain Spam allows level designers and developers create high volume and complex scenes with large amounts of objects in a much shorter amount of time.
– Optionally preserve prefab connections
– Select gameobjects to choose randomly from
– Automatically apply random or defined position, rotation, and scale
– Optionally snap to grid or intervals
– Auto assign a parent to created objects
– Layer masks for unwanted terrains
In a few steps you can begin placing many objects with a tap of the Spacebar.
You can also apply random settings and offsets to already created objects!